The present invention relates to a phase-shift keying (PSK) demodulator, and more particularly to a phase-shift keying demodulator for restoring digital data by reproducing a carrier and a clock signal from a signal modulated by PSK.
For demodulating a PSK signal, it is necessary to reproduce a carrier and a clock signal which are suppressed in principle.
There has heretofore been employed a quadruple system in which a received four-phase PSK signal is multiplied by four and then frequency-divided by four to erase phase information for thereby reproducing a desired carrier. The quadruple system is however liable to be affected by a data pattern contained in the received singal. Where data are received which are successive in the order of sin.omega.t, -cos .omega.t, -sin .omega.t and cos .omega.t, the frequency of the reproduced carrier tends to be shifted so that no stable carrier can be obtained.
When reproducing a clock signal from the detected signal for bit synchronization, it is difficult to generate stably a pulse signal having a duty cycle of 50% with respect to a data bit width, and therefore it is difficult to produce a stable clock signal with a phase comparator composed of a simple exclusive-OR logic circuit.